The present invention relates to sonar systems and more particularly to sonar systems employing two-dimensional scanner arrays to locate targets in three dimensions in a scanned volume.
A sonar which must scan a volume and locate targets in three dimensions requires at least a two dimensional array. Assuming that the two-dimensional array requires at least N elements in each dimension to achieve the desired acoustic beam shape, the total element count is as much as N.sup.2. A conventional Mill's Cross transducer configuration is often used to reduce the array element count to 2N because it only requires two line arrays of N elements each. One Mill's Cross line array is commonly a projector line and the other Mill's Cross line array is a hydrophone line so that the fan beam patterns of each line multiply to provide high resolution in two dimensions.
The fully populated array and the Mill's Cross array both have serious deficiencies when required to be focused to produce very high resolution. The projector can only be focused at one range and angle per ping. Consequently, the sector scan time becomes prohibitive with an increase in resolution. The sector scan problem may be mitigated by adding a second hydrophone line array parallel to the projector line. The projector may be shortened to insonify wide subsectors with adequate depth of field. The parallel hydrophone supplies the resolution with fewer elements. However, this technique is subject to signal masking in reverberant environments and often cannot be used.
A Mill's Cross array can be implemented with two independent orthogonal synthetic arrays thereby resolving the depth of field problem by enabling data to be collected before processing. The element spot count is the same as a real array for the same resolution and beam shape so 2N complex multiplies are required per resolution cell. The signal to reverberation ratio (SRR) in each beam can be low because of the wide beamwidth in one dimension for each array. The SRR problem is the same problem encountered when using wide projector beam and collinear hydrophone array described above. When two such orthogonal beams are combined, the increased beam resolution does not result in SRR increase.
A synthetic Mill's Cross can be implemented by scanning a projector element along a line orthogonal to a real line array of hydrophone elements. Backscatter data from each projector transmission is collected by all hyrdophone elements and combined linearly to give high beam resolution. This method works well in reverberant environments but requires N.sup.2 complex multiplies per resolution cell. It also requires a hydrophone array which may be impossible to implement at very high frequencies and resolutions. The problem is poor acoustic isolation and high noise levels of tiny closely packed hydrophone elements.
Accordingly, prior art, high frequency, high resolution, two dimensional arrays have been deficient in achieving sufficient SRR, fabricability, and data processing load.